Flameproofing agents are fire inhibitors which are intended to restrict, slow down or prevent the spread of fires.
Flameproofing agents are used wherever potential sources of ignition are located, or where the use of combustible materials constitutes a safety risk.
Rising demands for safety and the increasing use of high-grade plastics instead of metals and metal alloys, for example in the construction industry, aircraft and automobile manufacture and in interior fittings, have led to an increasing need for flameproofing agents.
The mode of action of flameproofing agents is based on different effects:                interruption of the radical chain reaction of the gases produced during pyrolysis of the material;        forming a protective coating of charred material (intumescence) in order to prevent the access of oxygen and heat;        cooling of the combustion process by initiation of an endothermic decomposition or evaporation of bound water;        dilution of the combustible gases by inert, gaseous substances;        liquefaction, i.e. formation of a melt which flows out of the fire zone and simultaneously reduces the surface area.        
Most flameproofing agents trigger one or more of the said chemical-physical effects:
Therefore the following four types of flameproofing agent are distinguished:                additive flameproofing agents—these are incorporated into the combustible substances;        reactive flameproofing agents—substances which are themselves components of the material by polymerisation into plastics;        inherent flameproofing agents—the material per se is flame-resistant;        coating—the flameproofing agent is applied as a coating onto the combustible substance from the outside.        
Important reactive and inherent, but also additive flameproofing agent have been criticised for toxicological reasons, i.e. formation of toxic gases during the decomposition process, and are now subject to a strict risk assessment so that inorganic flameproofing agents have gained increasing importance:
Worldwide production figures can be found, for example, in the report from the Danish Environmental Protection Agency (“Brominated Flame Retardants. Substance Flow Analysis and Assessment of Alternatives” (1999)). Accordingly the groupings of the flameproofing agents are broken down as follows:
50% inorganic flameproofing agents (IFA), for example ATH (Al(OH)3) and MDH (Mg(OH)2),
25% halogenated flameproofing agents,
20% organophosporus compounds,
5% nitrogen-based flameproofing agents.
In terms of quantity, aluminium hydroxide (ATH) is employed as the most important inorganic flameproofing. ATH is obtained from bauxite using the Bayer process. This produces red mud (RM) as a waste product. Therefore in the following description red mud (RM) is understood to be the residue from the Bayer process which is produced in the extraction of ATH from bauxite.
Red mud (RM), which may to some extent be represented as bauxite minus ATH, is an extremely heterogeneous substance (cf. Table 1), for example with regard to its chemical and mineralogical composition, its endothermic properties, its pH value, etc. The cause of the heterogeneity sometimes lies in the differing composition of the bauxites used, but above all in whether the Bayer process operates by autoclave digestion or by tube digestion. In the autoclave process the digestion is carried out with 30 to 35% caustic soda solution at temperatures of 170-180° C., so that a pressure of 6 to 8 bars is established. The tube digestion process was developed in order to shorten the reaction time of 6 to 8 hours to less than 1 hour by increasing the temperature to 270° C. However, at this temperature a water vapour pressure of 60 bars is established at the end of the reactor. The higher temperatures of the tube digestion also influence the composition of the red mud. For example, in the system Fe—O2—H2O in the tube digestion process the balance is shifted almost completely towards haematite.
In the past, because of the heterogeneity of the red mud (RM) no significant economic use could be found. In this respect red mud (RM) is almost exclusively disposed of as waste at disposal sites.